Cho tứ diện ABCD có thể tích bằng V hai điểm M,P lần lượt là trung điểm của AB,CD điểm \(N \in AD\) sao cho AD = 3AN. Tính thể tích tứ diện BMNP.
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Lời giải:
Báo saiTa có: \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOvamaaBa % aaleaacaWGbbGaamOqaiaadoeacaWGebaabeaakiabg2da9maalaaa % baGaaGymaaqaaiaaiodaaaGaamizamaabmaabaGaam4qaiaacUdada % qadaqaaiaadgeacaWGcbGaamiraaGaayjkaiaawMcaaaGaayjkaiaa % wMcaaiaac6cacaWGtbWaaSbaaSqaaiabfs5aejaadgeacaWGcbGaam % iraaqabaaaaa!49FA! {V_{ABCD}} = \frac{1}{3}d\left( {C;\left( {ABD} \right)} \right).{S_{\Delta ABD}}\) và \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOvamaaBa % aaleaacaWGqbGaamytaiaad6eacaWGcbaabeaakiabg2da9maalaaa % baGaaGymaaqaaiaaiodaaaGaamizamaabmaabaGaamiuaiaacUdada % qadaqaaiaadgeacaWGcbGaamiraaGaayjkaiaawMcaaaGaayjkaiaa % wMcaaiaac6cacaWGtbWaaSbaaSqaaiabfs5aejaad2eacaWGobGaam % Oqaaqabaaaaa!4A40! {V_{PMNB}} = \frac{1}{3}d\left( {P;\left( {ABD} \right)} \right).{S_{\Delta MNB}}\)
Dễ thấy \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamizamaabm % aabaGaamiuaiaacUdadaqadaqaaiaadgeacaWGcbGaamiraaGaayjk % aiaawMcaaaGaayjkaiaawMcaaiabg2da9maalaaabaGaaGymaaqaai % aaikdaaaGaamizamaabmaabaGaam4qaiaacUdadaqadaqaaiaadgea % caWGcbGaamiraaGaayjkaiaawMcaaaGaayjkaiaawMcaaaaa!483D! d\left( {P;\left( {ABD} \right)} \right) = \frac{1}{2}d\left( {C;\left( {ABD} \right)} \right)\)
Mặt khác \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4uamaaBa % aaleaacqqHuoarcaWGbbGaamOqaiaadseaaeqaaOGaeyypa0ZaaSaa % aeaacaaIXaaabaGaaGOmaaaacaWGKbWaaeWaaeaacaWGebGaai4oai % aadgeacaWGcbaacaGLOaGaayzkaaGaaiOlaiaadgeacaWGcbaaaa!4510! {S_{\Delta ABD}} = \frac{1}{2}d\left( {D;AB} \right).AB\) và \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4uamaaBa % aaleaacqqHuoarcaWGnbGaamOtaiaadkeaaeqaaOGaeyypa0ZaaSaa % aeaacaaIXaaabaGaaGOmaaaacaWGKbWaaeWaaeaacaWGobGaai4oai % aadgeacaWGcbaacaGLOaGaayzkaaGaaiOlaiaad2eacaWGcbaaaa!453C! {S_{\Delta MNB}} = \frac{1}{2}d\left( {N;AB} \right).MB\)
Mà \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamizamaabm % aabaGaamOtaiaacUdacaWGbbGaamOqaaGaayjkaiaawMcaaiabg2da % 9maalaaabaGaaGymaaqaaiaaiodaaaGaamizamaabmaabaGaamirai % aacUdacaWGbbGaamOqaaGaayjkaiaawMcaaaaa!4399! d\left( {N;AB} \right) = \frac{1}{3}d\left( {D;AB} \right)\) và \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamytaiaadk % eacqGH9aqpdaWcaaqaaiaaigdaaeaacaaIYaaaaiaadgeacaWGcbaa % aa!3BA6! MB = \frac{1}{2}AB\)
Do đó \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4uamaaBa % aaleaacqqHuoarcaWGnbGaamOtaiaadkeaaeqaaOGaeyypa0ZaaSaa % aeaacaaIXaaabaGaaGOnaaaacaWGtbWaaSbaaSqaaiabfs5aejaadg % eacaWGcbGaamiraaqabaGccaaMc8UaaGPaVlaaykW7daqadaqaaiaa % ikdaaiaawIcacaGLPaaaaaa!4914! {S_{\Delta MNB}} = \frac{1}{6}{S_{\Delta ABD}}\). suy ra \(% MathType!MTEF!2!1!+- % feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOvamaaBa % aaleaacaWGqbGaamytaiaad6eacaWGcbaabeaakiabg2da9maalaaa % baGaaGymaaqaaiaaikdaaaGaaiOlamaalaaabaGaaGymaaqaaiaaiA % daaaGaamOvaiabg2da9maalaaabaGaamOvaaqaaiaaigdacaaIYaaa % aaaa!4352! {V_{PMNB}} = \frac{1}{2}.\frac{1}{6}V = \frac{V}{{12}}\)
Đề thi thử tốt nghiệp THPT QG môn Toán năm 2020
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